s="" loss="" in="" the="" same="" decades="" was="" 3.7="" boys.Boys are also more than two-thirds more likely than girls to be bom prematurely before the 37th week of pregnancy. And, despite advances in public health, boys in the 1990s faced a 30 percent higher chance of death by their first birthday than girls; in contrast, back in the 1750s, they were only 10 per cent more likely than girls to die so early in their lives.Once they make it to childhood, boys face other challenges. They are more prone to a range of neurological disorders. Autism 自闭症 is notoriously higher among boys than girls: now nearly five times more likely, as tallied by the US Centers for Disease Control and Prevention. They are more susceptible than girls to damage from very low-level exposure to lead. Yet another problem: boys suffer from asthma at higher rates. There's also a stronger link between air pollution and autism in boys.Why do boys face such a burden of physical challenges? The answer is that the male’s problems start in the womb: from his more complicated fetal development, to his genetic makeup, to how his hormones work. The nine-month transformation from a few cells to an infant is a time of great vulnerability. Many chronic illnesses are seeded in the womb. In our species, the female is the default gender, the basic simpler model: Humans start out in the womb with female features (that is why males have nipples). The complicated transformation in the womb from female to male exposes the male to a journey packed with special perils. When the first blast of testosterone 睾酮 from the Y gene comes along at about the eighth week, the unisex brain has to morph into a male brain, killing off some cells in the communication centers and growing more cells in the sex and aggression centers. The simpler female reproductive system has to turn into the more complex male reproductive tract, developing tissues such as the testis and prostate 前列腺.Further, it takes a greater number of cell divisions to make a male; with each comes the greater risk of an error as well as the greater vulnerability to a hit from pollutants.On top of that challenge, the human male’s XY chromosome 染色体 combination is simply more vulnerable. The two XXs in the female version of our species offer some protection: In disorders where one X chromosome has a genetic defect, the female’s healthy backup chromosome can take over. But with his single X chromosome, the male lacks a healthy copy of the gene to fall back on. The X chromosome, which never shrank, is also a larger chromosome “with far more genetic information than the Y chromosome，’’ finds Irva Hertz-Piciotto, a University of California at Davis autism researcher, “so there may be some inherent loss of key proteins for brain development or repair mechanisms in boys.” This is a clue to the higher autism rate among boys, she asserts.Females also have a stronger immune system because they are packed with estrogen 雌激素，a hormone that counteracts the antioxidant process. “Estrogen protects the brain,” explains Theodore Slotkin, professor of neurobiology at Duke University's School of Medicine. “It repairs and replaces, even after neural injury,” Low estrogen even leaves boys more sensitive to head injuries. The male brain “is simply a more fragile apparatus, more sensitive to almost all brain insults,” says lead poisoning expert Herbert Needleman.It is the high levels of testosterone in the womb at critical times in gestation, according to British psychopathologist Simon Baron-Cohen, that are responsible for what he calls “the extreme male brain” -- the kind exhibited by autistic boys - low in empathy, high in systematizing. And in fact US researchers in recent decades have found unusually low estrogen and high testosterone levels among boys with autism.If the balance of hormones is out of whack in males, what made that happen? Researchers are coming up with some clues. In the New York City neighborhoods near Columbia University's Center for Children’s Environmental Health, families for years routinely sprayed their apartment
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